In cellular communications systems there are typically regions where the coverage is difficult or incomplete, for example, within metal-framed structures, or underground. Methods for improving the coverage in regions such as these are known in the art.
U.S. Pat. No. 5,404,570, to Charas et al, whose disclosure is incorporated herein by reference, describes a repeater system used between a base transceiver station (BTS) capable of receiving signals and a closed environment such as a tunnel that is closed off to transmissions from the BTS. The system down-converts high radio-frequency (RF) signals from the BTS to intermediate frequency (IF) signals. The intermediate frequency signals are then radiated by a cable and an antenna in the closed environment to a receiver therein. The receiver up-converts the IF signal to the original RF frequency. Systems described in the disclosure include a vehicle moving in a tunnel, so that passengers in the vehicle who would otherwise be cut off from the BTS are able to receive signals.
U.S. Pat. No. 5,603,080 to Kallandar et al., whose disclosure is incorporated herein by reference, describes a plurality of repeater systems used between a plurality of BTSs and a closed environment, which is closed off to transmissions from the BTSs. Each system down-converts an RF signal from its associated BTS to an IF signal, which is then transferred by a cable in the closed environment to at least one associated receiver therein. Each receiver up-converts the IF signal to the original RF frequency. Systems described in the disclosure include a vehicle moving between overlapping regions in a tunnel, each region covered by one of the BTSs via its repeater system. Thus, passengers in the vehicle who would otherwise be cut off from at least one of the BTSs are able to receive signals from at least one of the BTSs throughout the tunnel.
U.S. Pat. No. 5,513,176, to Dean et al., whose disclosure is incorporated herein by reference, describes a distributed antenna array within a region where reception is difficult. The performance of the antenna array is enhanced by generating signal diversity within the array. Each antenna in the array has a differential time delay applied to signals that it receives, thus generating received signal diversity. The differentially-delayed signals are preferably down-converted to an intermediate frequency and are then transferred out of the region via a cable.
U.S. Pat. No. 5,930,293, to Light, et al., whose disclosure is incorporated herein by reference, describes a wireless repeater comprising first and second spatially-separated antennas. Both antennas receive a signal from a transmitter, and the signal received by the second antenna has a time delay added to the original signal. The two signals are summed to form one aggregate signal, which is transmitted from a third antenna. A receiver of the aggregate signal is able to reconstruct the signals received by the first and second antennas.
Methods for dynamic allocation of elements of a cellular system are also known in the art. For example, U.S. Pat. No. 5,586,170 to Lea, whose disclosure is incorporated herein by reference, describes a system including a number of base stations that collectively define a service area. The base stations include associated directional antennas coupled to associated transmitters that serve sectors within associated cells. A mobile unit moves in the service area, and is capable of communicating with the base stations. A switch communicates with the base stations, and dynamically allocates channels to the base stations for use in associated sectors. The switch controls at least first and second base stations of different cells to serve communication with the mobile unit in associated sectors on the same channel. At least two base stations report associated signal strengths for communication with the mobile unit, to the switch.
U.S. Pat. No. 5,606,727 to Ueda, whose disclosure is incorporated herein by reference, describes a cellular communications system including a number of base stations which allocate channel time slots and frequencies for acceptable interference between base stations. Each base station divides its area into sectors served by corresponding antennae. Upstream interference levels in all directions are measured, preferably using an omnidirectional antenna. From these levels, candidate channels for assignment to mobile stations are determined. The base station indicates the candidate channel numbers to mobiles in its area. Each mobile measures downstream interference levels in the numbered channels and reports them simultaneously to making or receiving a call. Channels are assigned on the basis of reported levels.
U.S. Pat. No. 5,557,603 to Barlett et al., whose disclosure is incorporated herein by reference, describes communications apparatus for receiving a plurality of calls. The apparatus includes a plurality of receivers tunable to a plurality of frequencies, first and second antennas, switching means for switching the plurality of receivers to selected ones of the first and second antennas and control means coupled to the plurality of receivers for tuning the receivers to receive calls. The apparatus identifies a received call of low quality received through the first of the antennas and switches a spare receiver to receive the call through the second antenna.
German Patent 19,618,947 to Natarajan, whose disclosure is incorporated herein by reference, describes a dynamic channel allocation system for cellular communications. The system enables base station processing of subscriber unit request for channel allocation, or subscriber unit choice of most suitable base station